Do single matrix molecules generate primary ions in ultraviolet matrix-assisted laser desorption/ionization

Abstract

The ionization mechanisms in matrix-assisted laser desorption/ionization (MALDI) remain poorly understood. We have begun a program of study aimed at determining the properties of matrix molecules which make them suitable for MALDI. Initial results are presented here for one of the most widely used matrix materials, 2,5-dihydroxybenzoic acid (DHB). Spectroscopy of free DHB molecules in a molecular beam shows that the photoionization energy is much lower than expected from semiempirical calculations, only 8.05 eV, yet still not accessible with two nitrogen laser photons (7.36 eV). No evidence is found for labile protons in the first excited state. This is in spite of structural similarity with salicylates where excited state proton transfer from the 2-hydroxy group occurs. Conventional excited state proton transfer from single DHB molecules to analytes is thus deemed unlikely. On the other hand, a two-step reaction taking place via decarboxylated DHB (hydroquinone) is shown to be a potential analyte protonation mechanism. The conclusion is reached that single matrix molecules are probably not the primary ion generators in UV MALDI with this matrix. This is consistent with proposed models which require dimers or larger aggregates for ion generation. The photo/thermal combined ionization model of Allwood, Dyer and Dreyfus (Rapid Commun. Mass Spectrom. 11, 499 (1997)) is updated with measured physical parameters for DHB, and extended to include 2-center energy pooling mechanisms. © 1998 John Wiley & Sons, Ltd.